This is from the Sailrite website. I guess acid is what you need for rust.

There are a number of ways to clean sails. Some are more effective than others. Unfortunately, the most thorough methods also cause the most serious cloth breakdown which leads to greater stretch and, thus, a poorly setting sail. All mechanical methods fall into this category. Even large agitating tubs designed just for sails with carefully regulated water temperature will have the same effect on the sail as several weeks of hard use. And we definitely do not recommend using the machines at local Laundromats — they are never large enough and water temperature is not carefully controlled.


Most sailmakers recommend frequent rinsing with fresh water. If that is not sufficient, spread the sail on a smooth, clean surface and brush it lightly with a mild detergent solution or with a specially prepared cleaner such as “Sail-Kleen” or "Sail-Bath" from Davis. The latter is an enzyme detergent that can actually be used to soak away dirt and some stains.
Stains and spots can be removed as follows:


RUST. Soak the affected area in a 5% solution of oxalic acid dissolved in hot water. Follow this with a 2% solution of hydrochloric acid in warm water and, finally, rinse the spot well with clear water.


NUMBER ADHESIVE. Cleaner’s benzine used to be used here, but it is evidently rather dangerous and, as a result, very hard to obtain. Paint thinner and gasoline also work although they will leave an oil stain in some cases. My favorite is a proprietary product called GooGone that will not harm the fabric or stain it. It is available in most hardware stores and, of course, from Sailrite. We have also recently begun to stock "General Adhesive Remover" from 3M -- it works even better than GooGone. Do realize that two or three applications and some serious rubbing and scraping will be necessary no matter what you use.


OIL, TAR, VARNISH. Use trichloroethylene either by itself or in solution with a liquid detergent.


MILDEW. Soak the affected area in a 1% solution of chlorine (household bleach will do) and cold water. Vinegar also serves to control mildew if used rather frequently (it has limited use in correcting a problem that has gotten out of hand).


Whenever confronted with a serious stain, do not expect complete removal. Usually the best that can be expected is a moderation of the discoloration. After cleaning the sail, dry it completely. Mildew will not grow on modern synthetic sailcloth, but it can grow on the dirt that it accumulates if moisture is present. Although mildew will not harm the fabric, it will discolor or spot it.


Fold the sail and store it in a cool, dry place. The method of folding that you select is not very important, though try to keep folds to a minimum. There are several schools of thought regarding how this can best be done—choose whichever seems most convincing to you.